In pressurized-water nuclear reactors, the vessel containing the core of the reactor includes a substantially hemispherical head having openings in which there are fixed adapters allowing the passage of control rods of the reactor and instrumentation columns such as cylindrical thermocouple columns. A set of thermocouples is arranged in each of the thermocouple columns, making it possible to measure the temperature of the coolant fluid at the outlet of a set of assemblies of the core of the reactor.
Each of the adapters includes a part projecting under the head providing guidance for the thermocouple column, and a part projecting above the head including means for connecting a tubular assembly for supporting and sealing the thermocouple column, and which can be fixed into the extension of the adapter.
Inside the support assembly fixed to the adapter there is a shoulder against which a part of the thermocouple column comes to bear, with interposition of a sealing gasket.
The thermocouple column which passes through the bores of the support assembly and of the adapter, which are located in the axial extension of one another, includes an end which projects from the support assembly which is engaged with a device for clamping by pulling on the thermocouple column in the axial direction, so as to compress the sealing gasket between the thermocouple column and the support surface.
The device for sealed clamping by pulling on the thermocouple column may include, as described in FR-A-2,642,217, a pressure plate having a central opening making it possible to engage it over the end part of the thermocouple column, and including lifting screws each of which is engaged in a tapped hole passing through the pressure plate in a direction parallel to and distributed about the axis of the central opening. Each of the screws includes a first end projecting the plate, and coming to bear on the top part of the support assembly of the thermocouple column, and a second end projecting from a second face of the pressure plate which includes a shaped part making it possible to turn the lifting screw about its axis, inside the tapped hole. A two-part pull-ring may be interposed between the pressure plate and a radially projecting part of the thermocouple column, after engaging the pressure plate over the end part of the thermocouple column.
By rotating the screws using a manual tool, the pressure plate and, by means of the pull-ring, the thermocouple column, are raised, then the seal is clamped between the thermocouple column and the support surface of the support assembly.
In order to carry out effective and sealed clamping of the thermocouple column against the support surface, it is necessary to tighten each of the lifting screws to a specific constant torque for all the screws.
This clamping is generally carried out in two steps, a first clamping operation being performed before starting up the reactor, and a second complementary clamping operation being performed after a first rise in pressure inside the vessel of the reactor.
Four compression screws, placed at 90.degree. about the axis of the pressure plate, or preferably six screws placed at 60.degree. about the axis of the pressure plate, are generally used.
When six lifting screws are used, the time necessary for tightening them is of the order of 10 to 15 minutes, during which time the operator performing the manual tightening and monitoring the torque is exposed to the radioelement emissions coming from the contaminated surfaces of the head and of the adapters.
In addition, tightening any screw to a given torque changes the tightening torque of the adjacent screws. It is therefore very difficult to ensure uniform and satisfactory tightening of all lifting screws of the pressure plate and of the thermocouple column.
Furthermore, the use of an even number of screws does not allow the forces to be distributed properly over the periphery of the column, owing to the presence of pairs of screws in diametrically opposed positions.
Finally, the lifting screws bear directly on the end part of the support assembly which is securely fastened to the adapter. This results in conditions for taking up forces which can vary and which are not always optimal.